Sunday brings an end to the season – and possibly the series – following one of the Baker Street detective’s strongest chapters.

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The most shocking moment in the latest and perhaps most twist-filled episode yet of "Sherlock" didn't hinge on the revelation of a secret identity or a new look at an old piece of evidence.

No, the biggest stunner came when Holmes hugged Watson.

They stood in the reception room of 221B Baker St., two damaged men with little more to cling to than one another.

Both, at different points in the episode, used the same banal phrase to describe the dire circumstances driving their extraordinary bond: "It is what it is."

Nothing, of course, is what it seems on a show that thrives on the unexpected. The fourth season heads for a potentially explosive conclusion on PBS Sunday as Holmes battles an enemy who is, um, like no other.

Added drama looms past the closing credits: This could be, as star Benedict Cumberbatch suggested in a recent GQ UK interview, the final chapter of "Sherlock" (“It might be the end of an era,” he said).

Which would be a shame.

The show debuted in 2010, with its smart, modern take on Sir Arthur Conan Doyle's Victorian detective of deduction. Cumberbatch’s Holmes, a bursting bundle of insecurity and grandiosity (or, as Sherlock puts it, “a high-functioning sociopath”), needs Martin Freeman’s Dr. John Watson, a physically and psychologically battered war veteran who needs to be needed.

Short seasons of three episodes, paced about two years apart, proved worth the wait as “Sherlock” revivers Steven Moffat and Mark Gatiss worked on the similarly rebooted “Doctor Who” series, and Cumberbatch (“The Imitation Game,” “Doctor Strange”) and Freeman (“The Hobbit”) became movie stars.

The latest season’s first installment brought the (spoiler alert) murder of Watson’s wife, Mary, whose past as a secret agent came back to haunt her and whose demise split the Holmes-Watson team.

While uneven (Holmes' action-hero antics seemed out of character), the chapter set up the most recent episode, one of the series’ finest.

Toby Jones turned in a chilling performance as a rich businessman/celebrity with a serial killing habit on the side. He torments and nearly kills a drug-addled Holmes, who reunites in the nick of time with the life-saving Watson.

Their oddly affecting hug, though, gives way to a cliffhanger featuring a villain that's new (at least to the audience).

"Sherlock" is what it is, and what it is is great TV. Let's hope after Sunday’s episode, titled “The Final Problem,” that it’s not over.